The overall goal of the proposed studies is to gain insight into how phrenic motoneurons and diaphragm muscle (DIAm) fibers interact in the remodeling (plasticity) associated with conditions of altered use. Specifically, we will examine the role of phrenic motoneuron activity and the expression of specific neurotrophic factors (NT3 and NT4/5) and their receptors (TrkC and TrkB) in plasticity at DIAm fibers and neuromuscular functions (NMJs) under three experimental conditions of DIAm inactivity: 1) Unilateral denervation (DNV), where communication between phrenic motoneurons and DIAm fibers is complete disrupted, and motoneuron activity increases; 2) Unilateral tetrodotoxin (TTX) nerve blockade, where communication between phrenic motoneurons and DIAm fibers remains intact, and motoneuron activity increases; and 3) Spinal hemisection (SH) at C2, where communication between phrenic motoneurons and DIAm fibers remains intact, and motoneurons are inactive. In previous studies, we demonstrated that DNV and TTX are associated with a decrease in specific force of DIAm strips, a selective atrophy of type IIx and IIb DIAm fibers, and a decrease in the relative expression of MHC/2X and MHC/2B isoforms, w2hereas SH induced little change in DIAm fiber structure and function. In contrast, we found that in the SH DIAm, there is an expansion of NMJs innervating type IIx and IIb fibers. The proposed studies will extend these previous observations by evaluating the fiber type dependence of these adaptations and the role of selected neurotrophic factors. Specific factors: 1) To determine the influence of DNV and TTX on a) specific force, and 2) MHC content per half sarcomere of single DIAm fibers. 3) To determine the influence of DNV, TTX and SH on neurotrophic factor and receptor expression in a) DIAm, b) phrenic motoneurons, and c) NMJs. 4) To determine the effect of neurotrophic facto treatment on a) mechanical properties & MHC content of single DIAm fibers, and b) MHC expression and fractional synthesis rate of MHC isoforms in the DIAm. 5) To determine the effect of neurotrophic facto treatment on NMJ morphology and neuromuscular transmission.